The present invention relates to a novel method for manufacturing plastic products involving efficient use of heating and cooling. The invention relates to the preparation of plastic articles by methods which are much less energy intensive and more economical than those practiced in the prior art. The present invention is particularly successful in the manufacture of porous plastics but may also be adapted to molding processes wherein pressure on a softened, mobile plastic product is not required for part configuration or compaction. The present method is extremely attractive since it offers economy heretofore unavailable in existing practice.
Previous methods for casting and molding of thermoplastic or thermosetting materials have become very sophisticated, as evidenced by the following:
A. In compression molding, external heat is applied continually until the contained material melts and flows to cavity shape -- and if thermosetting, until cross-linking results.
B. In transfer and inJection molding, the materials are preheated to softening temperature, then impressed into a cool (if thermoplastic) or hot (if thermoset) mold.
C. In liquid or reaction inJection, heating or cooling is utilized depending upon the reaction (either exothermic or endothermic).
In the above described prior art processes, molds are complex and bulky, requiring internal channels to enable self contained heating and cooling. Thus, the common means for molding is very energy intensive because of the massive heat and cooling demands of the complex mold. The present process, using very light weight molds of highly heat conductive materials such as aluminum, makes it possible to heat and cool quickly. In the present process, the press platens stay hot at all times. The present invention also simplifies the equipment and its cost since heated presses are commercially produced in quantity and heat conductive molds are easily machined and replicated. No internal channels are needed.
Previous methods in the art of compression molding are described in the following U.S. Pat. Nos.: 2,373,201 to Smith; 2,521,282 to Butler; 2,532,501 to Johnson; and 2,812,549 to Wall. Each of these patents stipulates the use of heat and pressure during manufacture of plastic materials. The Smith patent describes a system in which heating and cooling of a platen is intrinsic and automatic.
In the present invention, pressure is not applied to the plastic during manufacture. The platens close for metal to metal contact only for efficient, rapid heating. Also, in the present invention, cooling does not take place in the same press, thus avoiding chilling of the rapid heating source. In the Johnson patent, heating is carried out electrically but cooling is accomplished with circulating water, again cooling the heat source as well as the molded part. Each of the four patents discloses the use of either piped heat or cold, requiring voluminous structures to permit drilling for the channels. It has also been found that channeling seldom yields uniform heat exchange due to poor conduction and geometry of the steel platen or molds.
The heating method of the present invention is deemed novel since the contact temperature is so high as to be undesirable, especially when the plastic product must not be melted. In a self-contained mold, such temperatures would ruin the product. Then the quick removal of the high heat source at a controlled lower temperature allows the lagging plastic temperature to reach the required thermal state.
The present method is quite different and is particularly useful in manufacturing products having critical surface and structural requirements. The method involves the use of a two-stage temperature control system, a platen press, and sophisticated timing devices. The preferred embodiments of the method are described below